I just recently fixed my custom PC(memory problem), but I was a few days late as I bought a extra HDD thinking it was a HDD problem. Stupid me.
Since I have 2 HDD now, I was thinking of using both. Is there any special way to do this? Or do I just plug in a sata cable and the power cable and I'm all set?

One possible problem is the extra HDD has part of the windows XP Setup on it, so I will need to reformat that also. How do I do this?

any help would be appreciated.

Just plug it in.

Go to Disk Management and Format it.

Right click My Computer then click Manage then Disk Management.